The present invention relates to coated cemented carbide products, and specifically, to means for improving the metal turning performance of such coated products.
Cemented carbides are well known for their unique combination of hardness, strength, and wear resistance and are accordingly extensively used for such industrial applications as cutting tools, drawing dies and wear parts. For abrasive wear and nonferrous metal-cutting applications, WC-Co compositions are preferred because of their high strength and good abrasion resistance. For steel machining applications, compositions consisting of WC-TiC-TaC-Co, TiC-Ni or Ti-Ni-Mo are preferred because they are less reactive with steel workpieces at high machining speeds. The use of carbides other than WC generally results in a significant strength reduction, however, which limits either the amount of TiC and other carbides that can be added or the severity of the application when large amounts of TiC are used.
It is known that the wear resistance of cemented carbides can be enhanced by the application of a thin coating of certain hard substances such as TiC and TiN. In this manner a better combination of toughness and metal-cutting wear resistance is obtained than can normally be obtained using uncoated compositions.
It is also known, e.g., from Austrian patent application No. 896/72, filed Feb. 4, 1972 and laid open to the public on May 15, 1973, that sintered carbide substrates can be provided with composite coatings comprising a first layer of titanium carbide and an overlying layer of titanium nitride, on the one hand, and a first layer of titanium carbide, an intermediate layer of titanium carbonitrides and an overlying layer of titanium nitride on the other hand.
A significant improvement in coated cemented carbides is reflected in U.S. Pat. No. 3,854,991 to Hale, issued Dec. 17, 1974 and entitled "Coated Cemented Carbide Products" the asignee of said patent being the same as that of the present invention. U.S. Pat. No. 3,854,991, which is incorporated herein by reference, discloses a coated cemented carbide product comprising a cemented carbide substrate and a fully dense hafnium or hafnium and zirconium carbonitride coating on the substrate. The coatings have a specific carbon-to-nitrogen ratio which enhances the performance of the carbide over such coatings as TiC and TiN.
It has now been discovered that a uniquely useful insert for cutting tools comprises a cemented carbide substrate on which has been deposited a first intermediate layer and then a hafnium carbonitride or zirconium carbonitride coating. More particularly, it has been found that underlayers of hafnium nitride, titanium nitride or zirconium nitride sandwiched between a cemented carbide substrate and a hafnium carbonitride coating enhances the performance of the insert. In addition, a specific two tier underlayer comprising a first layer of titanium nitride and a second layer of titanium carbide, overlying the titanium nitride layer, has been effective when sandwiched between the substrate and the hafnium carbonitride coating. When zirconium carbonitride coatings are used, underlayers of zirconium nitride, titanium carbide, or hafnium carbide have been effective.
It is an object of the subject invention to further enhance the toughness and wear resistance of known coated cemented carbides and to provide a process for effecting same.